The invention relates to operation of fossil fuel steam generators during start-up and at extremely low ratings, and in particular to control of heat absorption.
In a drum-type steam generating unit with superheat surface a total amount of heat absorption is required to heat the feed water to saturation temperature, to boil the water, and to superheat it to a desired level. Not only is there a requirement that the total heat absorption be supplied to the steam generator but the ratio of heat absorbed in the evaporative plus economizer surface to that absorbed in the superheater must be controlled. At high load operation various control methods have been successfully used including steam de-superheating, gas recirculation, and furnace burner tilt.
The same problem applies to reheat units whether of the drum-type or once-thru type, and it also applies to once-thru units with respect to superheat when the water wall outlet temperature is controlled for purposes of either limiting it to a safe value or for controlling the rate of temperature change during a start-up.
During start-up operation of a steam generator a minimum air flow in the order of 25 to 30 percent of the full load air flow is maintained through the steam generating unit. This air flow is introduced in the furnace to minimize the probability of a fuel rich flame failure due to insufficient oxygen in the furnace. With this high excess air, the gas side control methods used at higher loads are marginally effective and the superheating spraying is not effective at these low loads due to the lack of turbulence and evaporation of the spray at the extremely low stream flows occurring.